Solutions prepared from alkaline earth metal or alkali metals (“M”) and aluminum tend to precipitate over time. This problem is especially noticeable as the molar ratio of M:Al drops from 5:1 and approaches 1:1, where precipitation begins to occur at moderate temperatures less than 125° C. It is desirable that the feed of catalyst components to a melt phase polymerization process stay in solution to provide more uniform mixing with the reactants or polymer melt, and to enable feeding a consistent and uniform amount of desired catalyst to the melt phase process.
A catalyst solution has advantages over catalyst slurries, in that a solution avoids the potential for pumping and circulation problems, avoids transfer line fouling and plugging, and avoids the need for vigorous agitation used in slurries to prevent insoluble catalyst precipitates from settling in feed tanks. Precipitates in the feed tanks make feeding a uniform feed of catalyst to the melt phase polymerization process a problem.
Lithium hydroxide and aluminum isopropoxide can be combined in the presence of ethylene glycol to form a solution. This can be accomplished by heating the components to a temperature sufficient to form the catalyst in solution. The temperature for this reaction is normally in the range of 125° C. to 160° C. for one to five hours. Generally, the concentration of aluminum in the solution cannot exceed 3,000 ppm due to lack of solubility.
Precipitates can form under several conditions when a catalyst system is mixed in ethylene glycol. Precipitates can form when the catalyst solution cools down to ambient temperatures. Even when the composition remains as a solution upon cooling, over time (e.g. a matter of a two or three days at the lower Li:Al mole ratios) the solution can change to form precipitates. The amount of actual catalyst fed to a melt phase polymerization line for making the polyester through a feed system set at a given flow rate will fluctuate if precipitates form, thereby leading to inconsistent product types or product quality.
To maintain the catalysts in solution, an ethylene glycol/Li/Al catalyst composition must remain at an elevated temperature of about 150° C. or more when the molar ratio drops below 5:1, especially when the molar ratio is about 3:1 or less and approaches 1:1. Catalyst solutions maintained at high temperatures suffer from several disadvantages. Further, to maintain the catalyst solution at elevated temperatures requires increased plant capital for heated catalyst feed vessels. Another way precipitates form is when the amount of aluminum in the catalyst composition exceeds 3000 ppm. It is desirable to employ a catalyst feed source having a high concentration of Al so that the amount of solvent fed to the melt phase process can be reduced. Dilute catalyst systems can be used but suffer the drawback that a higher volume of the solution is fed to the melt phase process to meet a target catalyst metal content, thereby requiring the removal through evaporation or reaction of larger amounts of solvent from the melt phase process.
Not only can the catalyst precipitate in ethylene glycol solutions when 3000 ppm aluminum or more is used or if the hot solution is allowed to cool, but it can precipitate as the molar ratio of M:Al approaches 1:1. However, we have discovered that a molar ratio of M:Al of about 1:1 is desirable in some applications because the yellowness of the polyester polymer is minimized as the molar ratio of M:Al approaches 1:1.
Thus, it would be desirable to provide a catalyst composition which remains in solution at ambient conditions without agitation. Alternatively, or in addition, it would also be desirable if solutions can be made, if desired, at molar ratios of M:Al that approach 1:1 that are stable over a wide variety of temperatures, including ambient conditions. Alternatively, or in addition, it would be particularly advantageous if such solutions can be made using 3000 ppm Al or more to minimize the amount of solvent fed to a melt phase polycondensation process.